Changes in muscle size, architecture, and neural activation after 20 days of bed rest with and without resistance exercise

Yasuo Kawakami*, Hiroshi Akima, Keitaro Kubo, Yoshiho Muraoka, Hiroshi Hasegawa, Motoki Kouzaki, Morihiro Imai, Yoji Suzuki, Atsuaki Gunji, Hiroaki Kanehisa, Tetsuo Fukunaga

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

146 Citations (Scopus)


Nine healthy men carried out head-down bed rest (BR) for 20 days. Five subjects (TR) performed isometric, bilateral leg extension exercise every day, while the other four (NT) did not. Before and after BR, maximal isometric knee extension force was measured. Neural activation was assessed using a supramaximal twitch interpolated over voluntary contraction. From a series cross-sectional magnetic resonance imaging scans of the thigh, physiological cross-sectional areas (PCSA) of the quadriceps muscles were estimated (uncorrected PCSA, volume/estimated fibre length). Decrease in mean muscle force after BR was greater in NT [-10.9 (SD 6.9)%, P < 0.05] than in TR [0.5 (SD 7.9)%, not significant]. Neural activation did not differ between the two groups before BR, but after BR NT showed smaller activation levels. Pennation angles of the vastus lateralis muscle, determined by ultrasonography, showed no significant changes in either group. The PCSA decreased in NT by -7.8 (SD 0.8)% (P < 0.05) while in TR PCSA showed only an insignificant tendency to decrease [-3.8 (SD 3.8)%]. Changes in force were related more to changes in neural activation levels than to those in PCSA. The results suggest that reduction of muscle strength by BR is affected by a decreased ability to activate motor units, and that the exercise used in the present experiment is effective as a countermeasure.

Original languageEnglish
Pages (from-to)7-12
Number of pages6
JournalEuropean Journal of Applied Physiology
Issue number1-2
Publication statusPublished - 2001
Externally publishedYes


  • Countermeasure
  • Pennation angle
  • Physiological cross-sectional area
  • Twitch interpolation

ASJC Scopus subject areas

  • Orthopedics and Sports Medicine
  • Public Health, Environmental and Occupational Health
  • Physiology (medical)


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